Industrial Field of Application
The present invention relates to an automatic diaphragm control amplifier used with an objective for a closed circuit television (CCTV) camera.
Prior Art
In the prior art there has been no means provided for temperature compensation of a video signal level and, even if the level of a video signal actually varied due to a temperature variation, this video signal was usually processed by the built-in circuit arrangement of the camera as if the signal included no such level variation.
Problems to be Solved by Invention
In the camera employing pickup means other than a pickup tube such as a vidicon tube which is able to vary its sensitivity by changing its target voltage, this variation is often used in order to obtain optimal performance of said pickup tube or pickup element, so that a high quality picture of good S/N ratio is obtained for a bright scene and a high sensitivity takes preference over S/N ratio for a dark scene so that the function of the video gain control circuit incorporated in the camera is based on a brightness of the scene. This prior art camera has been inconvenient to use as an adequately bright scene would often result in a low quality picture of inferior S/N, since a level value set by changing the gain of the video amplifier is variable regardless of the brightness. For example, the video signal level may be lowered for the bright scene due to a temperature rise during shooting of this scene, and this signal level may function to increase the video amplifier gain of the camera as if the scene was darkened.
Furthermore, the pickup tube or the solid state pickup element is likely to be accompanied with increased dark current as a temperature rise resulting in that a portion of the picture which should be dark becomes bright. In addition to this tendency, lowering of the signal level due to the temperature rise may affect the function of the automatic diaphragm control amplifier to such an extent that an effective portion of the video signal is reduced and, in an extreme case, there will be no recognizable picture.